Telecommunication Networks Introduction

Transcription

1 Telecommunication Networks Introduction Telemedicina e e-saúde 2016/17 Pedro Brandão References These slides are the companions of Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009 With adaptations/additions by Rui Prior and Pedro Brandão 2 Telecommunication Networks Introduction 1

2 Digital, Analogic 3 Analogic Digital Sampling Quantification Images from Wikipedia 4 Telecommunication Networks Introduction 2

3 A few words on audio compression Audio signal sampling at a fixed rate: telephone: samples/sec CD: samples/sec Each sample is quantized, rounded: Ex.:, 2 8 =256 possible values Each quantized value is represented by bits 8 bits for 256 values example: samples/sec, 256 quantized values bps receiver converts bits back to analogue signal: some quality reduction Example rates CD: Mbps MP3: 96, 128, 160 kbps Internet telephony: 5.3 kbps and up 5 A few words on video compression video: sequence of images displayed at constant rate e.g. 24 images/sec digital image: array of pixels each pixel represented by bits redundancy spatial (within image) temporal (from one image to next) Examples: MPEG 1 (CD-ROM) 1.5 Mbps MPEG2 (DVD) 3-6 Mbps MPEG4 (often used in Internet, < 1 Mbps) Research: layered (scalable) video adapt layers to available bandwidth 6 Telecommunication Networks Introduction 3

4 Introduction 7 Introduction Our goal: get feel and terminology more depth, detail later in course approach: use Internet as example Overview: what s the Internet? what s a protocol? network edge; hosts, access net, physical media network core: packet/circuit switching, Internet structure performance: loss, delay, throughput protocol layers, service models 8 Telecommunication Networks Introduction 4

5 Summary 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 9 What s the Internet: under the hood PC server Wireless laptop Cellular phone Access points Wired links millions of connected computing devices: hosts = end systems running network apps communication links fibre, copper, radio, satellite transmission rate = bandwidth Mobile Network Global ISP Home Network Regional ISP Institutional Network router routers: forward packets (chunks of data) 10 Telecommunication Networks Introduction 5

6 What s the Internet: under the hood Network Equipment Hub (out-of-date) LAN Switches Access Point 11 Router Router What s the Internet: under the hood Network Equipment Firewall Modular server 12 Server Telecommunication Networks Introduction 6

7 What s the Internet: under the hood Data centre 13 What s the Internet: under the hood Data centre 14 Telecommunication Networks Introduction 7

8 What s the Internet: under the hood protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, Skype, Ethernet Internet: network of networks loosely hierarchical public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force Mobile network Home network Global ISP Regional ISP Institutional Network 15 What s the Internet: a service view communication infrastructure enables distributed applications: Web, VoIP, , games, e- commerce, file sharing, telemedicine communication services provided to apps: reliable data delivery from source to destination best effort (unreliable) data delivery Mobile network Global ISP Home network Regional ISP Institutional Network 16 Telecommunication Networks Introduction 8

9 What s a protocol? human protocols: what s the time? I have a question introductions specific messages sent specific actions taken when messages received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of messages sent and received among network entities, and actions taken on messages transmission, receipt 17 What s a protocol? a human protocol and a computer network protocol: Hi Hi Got the time? It s 14:00 TCP connection request TCP connection response GET <file> time 18 Telecommunication Networks Introduction 9

10 Summary 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 19 A closer look at network structure: network edge: applications and hosts access networks, physical media: wired, wireless communication links network core: interconnected routers network of networks 20 Telecommunication Networks Introduction 10

11 The network edge: end systems (hosts): run application programs e.g. Web, at edge of network client/server model client host requests, receives service from always-on server e.g. Web browser/server; client/server peer-peer model: minimal (or no) use of dedicated servers e.g. Skype, BitTorrent p2p c/s 21 Access networks and physical media Q: How to connect end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated? 22 1 Byte comprised of 8 bits Telecommunication Networks Introduction 11

12 Residential access: point to point Modem connection Uses existing telephony infrastructure Home is connected to central office up to 56Kbps direct access to router (often less) Can t surf and phone at same time: not always on DSL: Digital Subscriber Line Also uses existing telephone infrastructure Upstream and downstream bandwidth may be different rates depend on the technology, with VDSL2 can go over 100 Mbps dedicated physical line to telephone central office 23 Residential access: cable modems HFC: hybrid fibre coaxial asymmetric: up to 400 Mbps downstream, 20 Mbps upstream network of cable and fibre attaches homes to ISP router homes share access to router unlike DSL, which has dedicated access Usually supplied by cable TV companies 24 Telecommunication Networks Introduction 12

13 Cable Network Architecture: Overview server(s) cable headend 25 home cable distribution network (simplified) Typically 500 to 5,000 homes Cable Network Architecture: Overview FDM (more shortly): V I D E O V I D E O V I D E O V I D E O V I D E O Channels V I D E O D A T A D A T A C O N T R O L cable headend 26 cable distribution network (simplified) home Typically 500 to 5,000 homes Telecommunication Networks Introduction 13

14 Cable Network Architecture: fibre optics Active (AON) or passive (PON) Source: Images from Wikipedia 27 Local Area Networks local area network (LAN) connects terminals to routers Ex.: Ethernet: 10 Mbs, 100Mbps, 1Gbps, 10Gbps Usual configuration: terminals connect to Ethernet switch 28 Telecommunication Networks Introduction 14

15 Wireless access networks shared wireless access network connects end system to router via base station aka access point wireless LANs: b/g (WiFi): 11 or 54 Mbps n: up to 600 Mbps ac : up to 1300 Mbps wider-area wireless access provided by telecommunication operator ~14.4 Mbps over cellular system (EVDO, HSDPA) next up: LTE up to 300 Mbps over wide area Base station Mobile hosts Router 29 Home Networks 30 Typical home network components: DSL or cable modem router/firewall/nat Ethernet Wireless access point To/from central modem router/ firewall Ethernet Wireless laptops Wireless Access point Telecommunication Networks Introduction 15

16 Physical Media Bit: propagates between transmitter/receiver pairs physical link: what lies between transmitter & receiver guided media: signals propagate in solid media: copper, fibre, coax unguided media: signals propagate freely, e.g., radio Twisted Pair (TP) two insulated copper wires 31 Physical Media: coax, fibre Coaxial cable: two concentric copper conductors bidirectional Fiber optic cable: glass fibre carrying light pulses, each pulse a bit high-speed operation: high-speed point-to-point transmission (e.g., 10 s-100 s Gbps) low error rate: repeaters spaced far apart ; immune to electromagnetic noise 32 Telecommunication Networks Introduction 16

17 Physical media: radio signal carried in electromagnetic spectrum no physical wire bidirectional propagation environment effects: reflection obstruction by objects interference Radio link types: terrestrial microwave e.g. up to 45 Mbps channels LAN (e.g., Wifi) 11Mbps, 54 Mbps, 600 Mbps wide-area (e.g., cellular) 3G cellular: ~ 14.4 Mbps 4G cellular: ~ 100 Mbps Satellite Kbps to 45Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitude 33 Summary 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 34 Telecommunication Networks Introduction 17

18 The Network Core mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete chunks 35 Network Core: Circuit Switching End-end resources reserved for call link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required Establishing connection Data transfer disconnecting 36 Telecommunication Networks Introduction 18

19 Network Core: Circuit Switching Developed to transmit voice Intelligence resides on the network Dedicated resources per call No sharing (capacity wasted) Guaranteed performance For data Inactive line most of the time Dividing link bandwidth into pieces Frequency division (FDM) Time division (TDM) Good resource usage for voice calls Someone is always talking most of the time 37 Circuit Switching: FDM and TDM FDM Example: 4 users frequency time TDM frequency 38 time Telecommunication Networks Introduction 19

20 Numerical example How long does it take to send a file of bits from host A to host B over a circuit-switched network? All links are Mbps Each link uses TDM with 24 slots/s 500 ms to establish end-to-end circuit Mbps 24 64kbps bits 64kbps 10s + 500ms = 10,5s 10s * In practice we need to add propagation delay More on that later Network core: packet switching 40 Telecommunication Networks Introduction 20

21 Network Core: Packet Switching idea: divide end-to-end data flow in small pieces (packets) Packets of different users share network resources each packet uses full link bandwidth resources used as needed resource contention: aggregated resource demand can exceed amount available congestion: packets queue, wait for link use store and forward: packets move one hop at a time Node receives complete packet before forwarding Bandwidth division into pieces Dedicated allocation Resource reservation 41 Network core: packet switching Each packet is dealt with independently: No relation with preceding packets Each node chooses next hop per packet Packets for the same destination do not necessarily follow the same route (although they usually do) Packets may arrive out of order Destination re-orders them Packets may be dropped or corrupted in transit Source and destination are responsible for dealing with losses and corruption 42 Telecommunication Networks Introduction 21

22 Packet Switching: Statistical Multiplexing A Ethernet 100 Mb/s Statistical multiplexing C B queue of packets waiting for output link 1.5 Mb/s D E Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing. In TDM: each host gets same slot in revolving TDM frame. 43 Packet-switching: store-and-forward takes L/R seconds to transmit (push out) packet of L bits on to link at R bps store and forward: entire packet must arrive at router before it can be transmitted on next link Example: L = 7.5 Mbits (~0.94 Mbytes) R = 15 Mbps Transmission delay = 1.5 s delay = 3L/R (assuming zero propagation delay) 44 L R R R Telecommunication Networks Introduction 22

23 Packet switching versus circuit switching 1 Mb/s link Each user: 100 kb/s when active active 10% of time Packet switching allows more users to use network circuit switching: 10 users N users 1 Mbps packet switching : With 35 users, probability > 10 active at same time is less than Packets vs. Circuits Is packet switching a slam dunk winner? Great for bursty data resource sharing simpler, no call setup excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem 46 Telecommunication Networks Introduction 23

24 Internet: network of networks Internet is composed of Backbones: infrastructures to interlink networks, like NSFNET, in the USA, GÉANT in Europe, as well as commercial operators backbones like PT Regional networks, connecting for example universities and research institutes, e.g. FCCN Commercial networks, e.g. internal usage or for supplying services to customers, like internet connections, e.g. PT, ZON, VODAFONE Local networks 47 RCTS (Rede Ciência, Tecnologia e Sociedade) Education and research national network Newer diagram Communication platform to provide researchers, professors and students the specific network resources needed 48 Telecommunication Networks Introduction 24

25 GÉANT Pan-European network of research and education Interconnects RCTS and its 30 European counterparts Core at 100Gbps (with devel for 8Tbps), connections starting at 155Mbps 49 Internet: network of networks A packet flow crosses several networks Mobile network Global ISP Traceroute Monitis Visual Traceroute Domestic network Regional ISP Institution network 50 Telecommunication Networks Introduction 25

26 Summary 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 51 How do losses and delays occur? Packets queue in router buffers packet arrival rate exceeds output link capacity packets queue, wait for turn A Packet being transmitted (delay) B Packets queuing (delay) Free (available) buffer: arriving packets dropped (loss) if no free buffers 52 Telecommunication Networks Introduction 26

27 Four sources of packet delay 1. Node processing : Check bit errors Determine output link 2. Queuing time waiting at output link for transmission depends on congestion level of router A B Node processing Queuing 53 Four sources of packet delay 3. Transmission delay R= link bandwidth (bps) (rate) L= packet length (bits) Time to send bits into link = L/R 4. Propagation delay d = length of physical link (distance) s = propagation speed in medium (~2x10 8 m/s) A Note: s and R are very different quantities! transmission propagation Propagation delay = d/s 54 B Example Telecommunication Networks Introduction 27

28 Caravan analogy cars propagate at 100 km/hr toll booth takes 12 sec to service car (transmission time) car bit; caravan packet Q: How long until caravan is lined up before 2nd toll booth? Time to push entire caravan through toll booth onto highway = = 120 sec Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr A: 62 minutes 55 ten-car caravan toll booth 100 km 100 km toll booth Nodal delay d nodal d proc d queue d trans d prop d proc = processing delay Typically a few microseconds or less d queue = queuing delay depends on congestion d trans = transmission delay = L/R, significant for low speed links d prop = propagation delay A few microseconds to hundreds of milliseconds 56 Telecommunication Networks Introduction 28

29 Packet loss queue (buffer) preceding link has finite capacity packet arriving to full queue dropped (lost) lost packet may be retransmitted by previous node, by source end system, or not at all A Buffer (waiting area) Packet being transmitted Packet being propagated 57 B packet arriving to full buffer is lost Throughput Throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time server sends bits (fluid) into pipe pipe that can carry fluid at rate Rs bits/sec pipe that can carry fluid at rate Rs bits/sec 58 server, with file of F bits to send to client link capacity Rs bits/sec Rc bits/sec) Telecommunication Networks Introduction 29

30 Throughput Rs < Rc What is average end-end throughput? Rs bits/s Rc bits(s R s > R c What is average end-end throughput? Rs bits/s Rc bits(s bottleneck link 59 link on end-to-end path that constrains end-to-end throughput Throughput: Internet scenario Q: throughput per connection? min(rc,rs,r/10) R s R s R s in practice: Rc or Rs is often bottleneck R c R R c R c connections (fairly) share backbone bottleneck link R bits/sec Telecommunication Networks Introduction 30

31 Summary 1.1 What is the Internet? 1.2 Network edge end systems, access networks, links 1.3 Network core circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 61 Protocol layers Networks are complex!!!! Many pieces : hosts routers Links of various types applications protocols hardware, software 62 Telecommunication Networks Introduction 31

32 Sending snail mail Each layer uses the lower adding something: Address precision Extra services (reliability, receipt acknowledgement) Reception acknowledge Reception acknowledge Registered Registered Street, Number Street, Number Street, Number Postal Code Postal Code Postal Code Postal Code 63 Regional Dist. Local Dist. Why layering? Dealing with complex systems: explicit structure allows identification, relationship of complex system s pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layer s service transparent to rest of system e.g., change in gate procedure doesn t affect rest of system 64 Telecommunication Networks Introduction 32

33 Internet protocol stack Application: network applications FTP, SMTP, HTTP Transport: transferring data between processes TCP, UDP Network : routing of datagrams between source and destination IP, routing protocols Link: data transfer between neighbouring network elements PPP, Ethernet Physical: bits on the wire Application Transport Network Link Physical 65 message segment datagrama frame H l H t H n H t H n H t M M M M source application transport network link physical Encapsulation link physical switch 66 H l H t H n H t H n H t M M M M destination application transport network link physical H l H n H n H t H t M M network link physical H n H t M router Telecommunication Networks Introduction 33

34 The end Introduction to communication networks Telecommunication Networks Introduction 34